1. Technical Field of the Invention
The present invention relates to linear voltage regulators that are intended to provide a regulated voltage from a reference voltage and from an unstabilized supply voltage.
2. Description of Related Art
A linear voltage regulator generally includes a power element that operates in its saturated operating region and is connected in series with a load, to which it delivers a stabilized supply voltage Vout.
FIG. 1 shows a conventional example of a linear regulator. Such a regulator 1 is intended to supply a load Q. It generally comprises an MOS power transistor M intended to be connected in series with the load Q. In general, the transistor M is a pMOS transistor.
The transistor M is controlled in its saturated operating region by a regulating circuit POA, for example based on a differential amplifier that controls the gate voltage Vg applied to the transistor M. A first input, for example the inverting input of the regulating circuit POA, receives a reference voltage Vref and a second input, in the present case the non-inverting input, receives the output voltage Vout (this is the regulated voltage), drawn between the drain of the transistor M and the load Q. The voltage Vref is for example provided by a reference circuit capable of delivering a precise stable voltage. The amplifier POA is generally supplied by a voltage Vsup, for example provided by a battery.
Control of the transistor M is adjusted, by means of the voltage Vg applied to its gate via the output of the regulating circuit POA, as a function of the difference between the values received on the two inputs (inverting and non-inverting), so as to keep the output voltage Vout of the regulator constant.
It is useful in linear voltage regulators to limit the value of the current delivered by the transistor M. This makes it possible, when the current drawn by the load increases and consequently the output voltage Vout decreases, to prevent the output voltage Vg of the operational amplifier, which controls the transistor, from continuing to drop. This is because decreasing the voltage Vg increases the current delivered by the transistor M above a threshold value, running the risk of damaging both the load Q and the linear regulator.
Conventionally, as shown in FIG. 1, a resistor R is placed between the voltage Vsup and the source of the transistor. The voltage across the terminals of the resistor R, which is proportional to the value of the current delivered by the transistor M, is measured and compared by a circuit 3 with a specified voltage threshold. If the circuit 3 detects that the measured voltage is above the voltage threshold, the gate voltage Vg applied to the transistor M is modified so as to limit the current delivered by the transistor M.
However, this solution for limiting the current has the drawback of causing an undesirable voltage drop, and it may pose integration and heat generation problems.
Moreover, document US 2004/0178778 discloses a solution for limiting the current that does not cause such an undesirable voltage drop. This solution comprises the incorporation, into the operational amplifier, of a transistor, at the inverting input stage, and means which, above a certain current level delivered by the transistor, set the output voltage Vg of the amplifier POA that controls the transistor to a given value, thus stopping the current delivered by the transistor M from increasing.
However, because this solution is integrated into the voltage-regulating circuit (in the case disclosed: in the amplifier), it may be limited for regulating circuits that have an architecture similar to that in document US 2004/0178778. There is a need in art for a current limiting solution for use in a linear voltage regulator beyond that shown in the document US 2004/0178778.